Aircraft include complex fuel systems that route aviation fuel from fuel tanks to aircraft systems that use the fuel. In the case of an aerial refueling, these systems can also include the fuel lines, manifolds and associated valving necessary for delivering fuel to a receiver aircraft.
Aerial REFUELING hoses are equipped with couplings at their end to allow a proper fit of the receiver aircraft.
Standard couplings are in accordance with the standard MIL-PRF-81975 and the historical standard MIL-C-81975. The current state of the art is based on MA-4 couplings. Those couplings have the mechanical interfaces defined by the military standard and, in general, comply with the performance requirements.
In terms of pressure regulation, current MA-4 couplings are equipped with two regulators which are located in series in a channel. The main regulator is exposed to the outlet or delivery pressure such that it is the regulator responsible for supplying said required outlet pressure. The other regulator or auxiliary regulator helps the main regulator in supplying high flows providing an additional pressure in the main regulator such that the main regulator is able to avoid its throttle.
The main regulator moves depending on the outlet pressure because it is directly exposed to it. When the pressure difference between the ambient pressure, measured through a port, and the delivery pressure exceeds a target value, the main regulator closes and when the difference is less, it is opened. Therefore the main regulator balances its position based on the balance of pressures on a restricting element.
The auxiliary pressure regulator is located before the main regulator so it is not exposed to the delivery pressure. The auxiliary pressure regulator, depending on the flow and on the inlet pressure, increases the downstream pressure to conform the operation range of the main regulator, preventing its throttle.
In fact, both regulators are needed to supply the required pressure. A drawback of current MA-4 couplings is that they are not able to independently achieve the delivery pressure requirement, this meaning that in the case of one pressure regulator failure, the system might not deliver the fuel at the required pressure causing potential damages on receiver pipelines.
Therefore, said coupling is not redundant as the working of both regulators is not independent and moreover in case of single failure the coupling is not able to properly regulate the delivery pressure.
Another drawback is that the failure of one of the regulators is a hidden failure only detected during routine maintenance tasks.
On the other hand, the high pressure drop of the current designs for MA-4 coupling is penalising the system performances due to narrow passages and sharp edges in the interior walls. Preliminary issues of the standard MIL-PRF-81975 specified lower values for the acceptable pressure drop which had to be increased to allow the fulfillment by the industry.
Currently there is a need in industry to have new couplings which ensure a full dual pressure regulation, however no system has been developed yet to ensure full redundancy on the pressure regulation system components.